Semiconductor devices include insulating films made of a material, such as SiO2, PSG (Phospho Silicate Glass), P—SiO (formed by plasma CVD), P—SiN (formed by plasma CVD), SOG (Spin On Glass), Si3N4 (silicon nitride), etc. Particularly, silicon nitride films are widely used, because they have better insulation properties as compared to silicon oxide films, and they can sufficiently server as etching stopper films.
Some methods are known of forming a silicon nitride film on the surface of a semiconductor wafer by thermal CVD (Chemical Vapor Deposition). In such thermal CVD, a silane family gas, such as monosilane (SiH4), dichlorosilane (DCS: SiH2Cl2), hexachlorodisilane (HCD: Si2Cl6), or bistertialbutylaminosilane (BTBAS: SiH2(NH(C4H9)2), is used as a silicon source gas. For example, a silicon nitride film is formed by thermal CVD using a gas combination of SiH2Cl2+NH3 (see Jpn. Pat. Appln. KOKAI Publication No. 2-93071) or Si2Cl6+NH3.
Owing to the demands of increased miniaturization and integration of semiconductor integrated circuits, insulating films as those described above need to be made thinner. Furthermore, in order to maintain the electric properties of the various films that lie below insulating films, the temperature used in thermal CVD in forming the insulating films needs to be lowered. In this respect, for example, where a silicon nitride film is deposited by thermal CVD, a high process temperature of about 760° C. is conventionally used. In recent years, where a silicon nitride film is deposited by thermal CVD, a process temperature of about 600° C. is used, as the case may be.
Where semiconductor devices are fabricated, conductive films and insulating films as described above are stacked one on the other, and are subjected to pattern etching to form a multi-layer structure. When an insulating film is formed and then another thin film is formed thereon, contaminants such as organic substances and particles may have stuck to the surface of the insulating film. Accordingly, a cleaning process is performed to remove the contaminants, as needed. In this cleaning process, the semiconductor wafer is immersed in a cleaning solution, such as dilute hydrofluoric acid, to perform etching on the surface of the insulating film. Consequently, the surface of the insulating film is etched by a very small amount, thereby removing the contaminants.
Where such an insulating film is formed by CVD at a higher process temperature of, e.g., about 760° C., the etching rate of the insulating film during the cleaning process is very small. Accordingly, the insulating film is not excessively etched by cleaning, and thus the cleaning process is performed with high controllability in the film thickness. On the other hand, where such an insulating film is formed by CVD at a lower process temperature of, e.g., about 600° C., the etching rate of the insulating film during the cleaning process is relatively large. Accordingly, the insulating film may be excessively etched by cleaning, and thus the cleaning process entails lower controllability in the film thickness.
Further, a silicon nitride film may be used as an etching stopper film. In this case, the silicon nitride film is required to have a very small etching rate. However, the conventional film formation method cannot satisfy this requirement.